The floor in aircraft, in particular in cargo aircraft, has to meet increased mechanical requirements. The generally substantially planar and continuous floor is formed by a plurality of floor panels adjoining one another.
The floor construction is exposed, inter alia, to high mechanical loads caused by the cargo itself. High local surface pressures generally occur, caused by trolleys, the tyres of vehicles and dirt particles that may be pressed into the surface of the floor by tyres, rollers or chain links. In addition, increased attention has to be given to the so-called “impact” behaviour. This is so since handling loads, chains or tools that fall on the floor must not adversely affect the integrity of the floor.
Furthermore, the great temperature differences occurring during flight operation, of up to 125° C., must not produce any appreciable mechanical stresses within the floor. To achieve this, the floor panels are connected to the floor substructure or the fuselage cell by suitable connecting elements, which allow minor movements in at least one direction parallel to the plane of the floor (known as “floating” mounting of the floor).
In addition, the floor panels also have a bearing function. This is so since the floor panels have the task of transferring loads, for example from lashing points in the floor by way of shearing forces into the lateral connection to the skin of the fuselage cell, as a result of which their displaceability, and consequently their length, is limited. Together with channels in the floor, which serve for receiving so-called “lashing points”, roller conveyors, guide rails or the like, the floor panels form a statically effective assembly and increase its overall flexural rigidity. The floor panels consequently also define the optimum load introduction point in the x direction, that is to say parallel to the longitudinal axis of the aircraft, for loads that are fastened on the floor by means of the lashing points.
On account of the required displacability of the floor panels to compensate for thermal expansions, the floor panels do not act as continuous beams, but can only be designed as beams on two supports. A weight-optimized solution for this is represented by the “beam of equal stress”.
Such a beam can be produced, for example, in the form of an integrally milled panel. The machining effort is in this case very great. In addition, such a beam cannot be produced with optimum weight by a milling technique.
Alternatively, the floor panels may be formed by sandwich panels, the outer layer of which is formed by metal. When there are high local loads and great supporting widths, the weight advantage is no longer obtained. In addition, the impact resistance when high mechanical point loads occur is problematic. Delaminations caused by the impact of compact objects are among the problems that are difficult to detect, as a result of which weight-increasing safety margins have to be allowed for in the static design.